1. Field of the Invention
The present invention relates generally to electromagnetic energy field transmission and detection systems, and more particularly to an apparatus for transmitting a high intensity uniform electromagnetic field and simultaneously detecting a localized low-intensity electromagnetic field.
2. Brief Discussion of the Prior Art
Many objects, such as houses, pets and cars, require some means of identification. Many prior methods of identification have required visual interrogation of the identifying medium to extract the identification data, such as reading numbers on houses, license plates on cars, and collar tags or brands on animals. Electronic identification tags have also been created, which can be associated with the object and electronically communicated with at a distance, such as the electronic sensing and actuator systems shown in U.S. Pat. Nos. 3,732,465 and 3,752,960.
The systems described in those patents are comprised of an active element, having a single transmitting and receiving coil, which operates by transmitting an electromagnetic field within the proximity of a passive electrical circuit, thereby inductively coupling with the passive circuit. When the passive circuit is inductively coupled with the coil, a characteristic change in the electromagnetic field is created, which is then detected by the electronics associated with the receiving coil. Although such systems remove some of the restrictions associated with the previously described visual identification systems, such system are really nothing more than electronic keys, and actually convey less information to the active element than does a visually inspectable tag.
More sophisticated electronic systems transmit a high intensity electromagnetic energy field to an electronic tag or transponder, which is thereby energized by the electrical energy carried by the magnetic field and made operative to output an identification signal which can be detected by a remotely located receiving coil. The passive tag or transponder element of some of these devices, such as described in U.S. Pat. No. 4,262,632, typically include a capacitor, which collects energy transmitted by the transmitting coil and then outputs power to the identification circuitry. Systems which utilize a sufficiently large capacitor or resident power source, such as a battery, are able to transmit identification signals over distances as large as a few yards. However, the packaging which is required to house a battery or capacitor results in the transponder being generally too large for many applications, such as identifying small animals.
In order to increase the potential utility of such electronic identification systems, recent efforts have been made to decrease the size of the transponding unit, such that it may be syringe-implanted within small animals, such as is disclosed in European Patent No. 258,415. This patent and other patented systems, such as U.S. Pat. Nos. 3,859,624, 3,689,885, 4,532,932 and 4,361,153, disclose passive elements which operate in real time and therefore do not require any type of significant energy storage means. The transmitting and receiving units of the systems, which are often referred to as interrogators or readers, typically include either a single, dual or triple coil arrangement, which is used to both transmit a high intensity electromagnetic field to the identification unit and receive an identification signal reradiated by the transponder in a low intensity electromagnetic field.
The detection range of such systems is typically very restricted since the strength of the electromagnetic field produced by the transponder drops in strength by 1/d.sup.5, where d is the distance between the receiving coil and the transponder coil, as the interrogator is moved away from the transponder. In fact, it has been calculated that at a distance of 6 to 8 inches the magnetic field carrying the identification signal from the transponder, in devices similar to that described in the European Patent No. 258,415, have only an estimated one billionth the strength of the magnetic field carrying the interrogation signal to the transponder. Hence, the interrogator must normally be placed in very close proximity to the transponder in order to detect the identification signal carried thereby. This limitation, of course, greatly restricts the utility of such devices, since not all objects may be so closely approached in order to be read.
Although the specification of European Patent No. 258,415 states that the system disclosed therein is operative to detect the retransmitted signal at distances on the order of inches, it has been found that devices constructed in accordance with that specification are actually incapable of obtaining such a range and are generally only effective when positioned within about an inch of the transponder. An identification device which is limited to within such a small range of operability is of limited usefulness, especially when it is desired to be used to identify large or wild animals or other objects which cannot be readily approached. In addition, such devices are also highly susceptible to interference and noise produced from other sources which affect the integrity of the detection portion of the system. This latter problem with such identification systems also increases in severity as the transponder is moved away from the interrogator, since it becomes more and more difficult to distinguish between the high intensity transmission field and the low intensity transponder field as the transponder field drops in strength.
Hence, a need has arisen for a transmission and detection system which can simultaneously transmit a high energy magnetic field, sufficient to power the transponder unit, and detect a localized retransmitted magnetic field at greater distances and with greater reliability.